An Overview of 30 Years of AREVA TN’s Burnup Credit Practices for Transport and Interim Storage of Used Fuel Assemblies

Credit for criticality safety evaluation from the decrease in reactivity during fuel irradiation due to the change in concentration (reduction) of fissile nuclides and the production of neutron absorbing nuclides is commonly referred to as BUrnup Credit (BUC). The use of the BUC approach for transport and dry storage cask applications requires careful considerations of the fuel operating history, validation of calculational methods used for the prediction of isotopic compositions after irradiation for criticality assessments, and also considerations of additional measures in order to ensure safe cask loadings.Since the early nineteen eighties, AREVA TN has implemented BUC methods for the demonstration of the sub-criticality of transport and interim storage casks loaded with PWR uranium oxide (UO2) used fuel assemblies. Initially, a simplified BUC method based on the sole consideration of only major actinides and the use of a partial burnup credit was applied. As long as the fuel enrichment of the PWR UO2 fuel assemblies is sufficiently low, this simplified BUC approach is  satisfactory to cover industrial needs without the necessity of designing new casks or of increasing the amount of neutron absorbers in new basket designs. Nevertheless, the ever-increasing enrichment of PWR UO2 fuel assemblies over the last decade has led to the development and then to the implementation of an advanced BUC methodology including not only actinides but also major fission products. This advanced BUC method enables AREVA TN to limit both the increase in the neutron poison content in new basket designs as well as the increase in the performance of the casks with the existing baskets without physical modifications. A big step forward has recently been taken by AREVA TN who has obtained, for the first time, approvals from French and German Competent Safety Authorities for the application of this advanced BUC approach to initially highly enriched PWR UO2 used fuel assemblies.